User Manual

How the Cable Modem Router Implements the Port
Forwarding Rule
The following sequence shows the effects of a port forwarding rule:
1. When you type the URL www.example.com in your browser, the browser sends a
web page request message with the following destination information:
•
Destination address. The IP address of www.example.com, which is the address
of your cable modem router.
•
Destination port number. 80, which is the standard port number for a web server
process.
2.
Your cable modem router receives the message and finds your port forwarding rule
for incoming port 80 traffic.
3. The cable modem router changes the destination in the message to IP address
192.168.0.123 and sends the message to that computer.
4. Your web server at IP address 192.168.0.123 receives the request and sends a reply
message to your cable modem router.
5.
Your cable modem router performs Network Address Translation (NAT) on the source
IP address, and sends the reply through the Internet to the computer or mobile
device that sent the web page request.
Set Up Port Triggering
Port triggering is a dynamic extension of port forwarding that is useful in these cases:
•
An application must use port forwarding to more than one local computer (but not
simultaneously).
•
An application must open incoming ports that are different from the outgoing port.
With port triggering, the cable modem router monitors traffic to the Internet from an
outbound “trigger” port that you specify. For outbound traffic from that port, the cable
modem router saves the IP address of the computer that sent the traffic. The cable
modem router temporarily opens the incoming port or ports that you specify in your
rule and forwards that incoming traffic to that destination.
Port forwarding creates a static mapping of a port number or range of ports to a single
local computer. Port triggering can dynamically open ports to any computer when
needed and close the ports when they are no longer needed.
User Manual95Specify Internet Port Settings
AC1750 WiFi Cable Modem Router